by Emilio (fresno ca.)
If a piece is checking the king, must the king move or can the piece doing the checking be eaten by a piece that is in position to do so?
Both possibilities exist. You can move the king away from the check or capture the piece that is doing the checking, if this is possible and advisable.
In the position above the black king is in check and Black has two choices:
1. To move the king away or
2. To capture the knight with the rook (RxN)
Both moves are technically possible but you have to evaluate what is best.
The best move is to go away with the king (1…Kf6).
Because when you capture the knight with the rook you lose rook for knight (1…RxN 2.hxR).
This means you have lost material as a rook has a higher value than a knight.
The value of a rook is 5 pawns.
The value of a knight is 3 pawns only.
For that reason it is advisable to move the king to e7 or f6 to get out of the check.